In the anode sludge which results from the electrowinning of copper and the electrolytic refining of copper, there are present varying amounts, depending upon the source of the anode, of silver, gold, platinum, palladium, selenium, tellurium, arsenic, antimony, copper, nickel, lead, etc. The main purpose of a treatment of the anode sludge is, therefore, the separation and recovery of valuable materials therefrom. Various methods have been proposed for this purpose in the past.
In the usual processes, the sludge is pyrometallurgically treated with a flame in the drive hearth process and the resulting anode silver is further electrochemically treated.
This process has a number of significant drawbacks. It is especially labor intensive and energy intensive and it is necessary to use slag charges which take up much of the noble metals and require separate processing of the slags to recover the noble metals.
More recently, processes combining wet chemical and pyrometallurgical process steps have been described.
For example, U.S. Pat. No. 4,002,544 and German Patent Document DE-AS 2543027 describe a process in which decoppered anode sludge is subjected to a sulfidizing roasting step at about 160.degree. C. to 300.degree. C. to decompose the copper and silver selenides and tellurides. The product is then leached with hot sulfuric acid in which about 95% of the silver, selenium and tellurium are dissolved.
From the filtered salt sludge, nickel is leached by hot water and the residual sludge is treated by other conventional processes to recover the gold, platinum and palladium.
After dilution of the sulfuric acid filtrate, the silver, selenium and tellurium are cathodically recovered by electrolysis and the resulting metal powder is smelted with a supply of air during the smelting process to volatilize the selenium and tellurium as the respective oxides. The metal is then supplied as anode silver to a silver electrolysis stage. This process avoids the smelting treatment in the flame furnace but it requires an additional electrolysis step for the separation of the silver, selenium and tellurium. Aside from this, the sulfidizing roasting does not provide a precise separation of the components.
In another process described in German patent Document DE-As 146712, the anode sludge, freed from selenium and tellurium, is subjected to a treatment in which the silver, copper and lead are solubilized as the nitrates and are processed in a silver electrolysis step. From the sludge residue the total gold, platinum and palladium are recovered by one of the standard pyrometallurgical or hydrometallurgical processes.
The silver contained in the nitric acid solution is predominantly recovered by an electro-winning process and the remaining silver can be recovered by cementation with copper. From the desilvered solution the lead is precipitated with sulfuric acid and the lead sulfate and is filtered off. The filtrate, for separation of the nitric acid and sulfuric acid, is distilled and rectified and the crystallization residue in the sump of the still is dissolved in water and worked up to recover copper or copper salts.
When palladium is present in the anode sludge, this process is not suitable because palladium is largely soluble in nitric acid and a separate step for recovery of this valuable metal must be carried out.
A further disadvantage is that the high palladium and copper contents can require very costly electrolytic process which may make the overall process completely uneconomical.
Another process which requires consideration (see German Patent Document DE-OS 2117513) provides direct chlorination of the anode sludge from which lead has previously been separated.
The anode sludge is mixed with dilute hydrochloric acid to form a low viscosity slurry and during agitating gaseous chlorine is introduced at a temperature of about 100.degree. C. With the exception of silver, the chlorination results in the solubilization of all of the materials in the sludge. The slurry is then hot filtered and hot washed to remove the principal proportion of the lead as the lead chloride.
The silver chloride is then extracted with ammonia to separate silver from the residual accompanying elements antimony, tin and silica. The recovery of silver from the ammonical solution is effected by evaporating the ammonia leaching silver chloride precipitate with sodium hydroxide, reducing the resulting Ag.sub.2 O with reducing sugars to a pure powdered silver metal and smelting the latter powder.
A drawback of the use of chlorine gas in a solution which can have a normality of 12N of hydrochloric acid, corresponding to 432 g of a hydrogen chloride per liter at 100.degree. C. is that the process may be dangerous and creates corrosion problems. It requires reflux cooling for the hydrogen chloride vapors and creates problems with the separation of the PbCl.sub.2 from the sludge because of the temperature drop in the filter press and the plugging of the pipe lines associated therewith. A precise separation of the lead and silver is not possible.
The German Democratic Republic Patent No. 201,920 describes the leaching of gold and silver from the silver ores with low noble metal contents using aqueous sodium chloride solutions (200-250 g/l). The ore either before leaching or during leaching is aerated possibly in conjunction with the addition of Cu.sup.++. This process can be supplemented by the addition of hypochlorite ions during the leaching when gold is present and the process is then carried out at a pH value above 4.
The oxidation is then effected by the hypochlorite ion which decomposes in this pH range into the chloride and oxygen in accordance with the relationship: EQU NaOCl.fwdarw.NaCl+O
U.S. Pat. No. 4,439,235 also describes the leaching of ores utilizing said hypochlorite in which the leaching is effected by decomposing the ore with solutions which contain about 500 g of sodium hypochlorite per liter and about 20 ml of HCl/l. The addition of hydrochloric acid here serves only to set the pH value of the solutions at about 4 to 5. The oxidation is here accomplished by the decomposition of the sodium hypochlorite to the sodium chloride and oxygen as described.
There is no direct in situ generation of nascent chlorine gas in the slurry suspension described or suggested in this disclosure.
In EP-A10 176 100, the use of sodium hypochlorite to generate chlorine gas is exploited in that the sodium hypochlorite solution is introduced into a hydrochloric acid slurry which has previously been freed from acid soluble copper, nickel and lead.
Of course, this process requires a separate step for the removal of the copper, nickel and lead.